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The two games are very different though, of course, they both deal with public transport.

The original game runs on an engine of CO's design. Only straight streets can be used. The game heavily relies on campaigns and scenarios. It's a lot of fun to complete the campaigns, but once you've done them, it's pretty much game over. You can go back and do them again, but it's not as much fun the second time around. It's also a very pretty game with nicely done buildings and maps.

CiM2 uses the Unity engine that allows for curved streets and metro rail. It's a much more complicated and involved game as it requires you to set up timetables. It too has scenarios (including user created ones; it has a built in scenario editor) which are also fun to complete. But, unlike the original game they have more "replayability". Unfortunately, CiM2 was never finished. We were promised a patch for some omissions and quirks in the game, but when SimCity 2013 flopped CO began working on Cities:Skylines. Even so, it's a good game as it is.

You won't go wrong buying both games as they are different. Both games have a lot of DLCs to buy that give you additional vehicles, maps and scenarios; so I'd wait until they're on sale. But, if the price doesn't bother you, go for it; you'll get your money's worth.
 
imo CiM is better. I launched CiM2 and couldn't even complete the tutorial because the text is only legible if you play on something lower than 1280x1024 resolution because it is tiny to begin with and than it doesn't scale so it become even tinier.